CHARLESTON, S.C. — Under duress two days before the pivotal South Carolina primary, Republican front-runner Mitt Romney acknowledges he has part of his millions invested in the Cayman Islands, although his aides say he never used the location as a tax haven.

Neither Romney nor his campaign are providing details, including how much he has invested there, or why, or if any of his money is invested elsewhere outside the United States.

"Gov. and Mrs. Romney's assets are managed on a blind basis. They do not control the investment of these assets," campaign spokeswoman Andrea Saul said in an emailed response to questions Wednesday night. Romney has not personally addressed the issue, although it's likely to come up at a candidates' debate Thursday night.

Scrutiny of Romney's private-sector background couldn't come at a worse time — just as he is riding high on back-to-back victories in Iowa's caucuses and New Hampshire's primary. He came into South Carolina with anti-Romney conservatives unwilling or unable to coalesce behind a single challenger.

Details about the personal fortune of the multimillionaire former businessman have been a nagging issue for his campaign for more than a week. On Tuesday, Romney disclosed that he pays an effective tax rate of about 15 percent, lower than what he would pay if he earned a regular paycheck like many Americans. He also called "not very much" the amount he earned in speechmaking fees, though it turned out to be $373,327.62 for 12 months in 2010 and early 2011.

Those details followed verbal miscues related to his views of work. Last week in New Hampshire, Romney told an audience he knew what it was like to worry about being "pink-slipped" and losing a job. A day later, he said, "I like being able to fire people who provide services to me" — a comment about health insurance companies that his rivals used to paint a picture of Romney as a wealthy businessman who is out of touch with ordinary Americans.

Romney hasn't been willing to elaborate on his pledge Tuesday to release his federal tax return in April. His rivals are prodding him to do it immediately, before South Carolina's primary, a potentially decisive turn in the race for the Republican presidential nomination.

At an event in Rock Hill, S.C., Romney kept away from the issue of his taxes, but he criticized Newt Gingrich and other Republicans who "jumped on that bandwagon" of criticizing free enterprise. "My goodness, I listened to Speaker Gingrich the other night talk about the enterprises I've been associated with," he said. "I'm proud of the fact that I worked in the private sector, that I've achieved success."

Romney disclosed for the first time Tuesday that, despite his wealth of hundreds of millions of dollars, he has been paying taxes in the neighborhood of 15 percent, far below the top maximum income tax rate of 35 percent, because his income "comes overwhelmingly from investments made in the past." During 2010 and the first nine months of 2011, the Romney family had at least $9.6 million in income, according to a financial disclosure form submitted in August.

Romney had been consolidating GOP support before Saturday's South Carolina primary, in which a victory could all but seal his nomination. The focus on his wealth is an unwanted distraction for him as he seeks to win votes in a state where the unemployment rate, at 9.9 percent, is among the highest in the nation, and amid rising public concern over income inequality.

President Barack Obama's campaign advisers contend voters are unlikely to back a wealthy Republican with financial-industry ties at a time of lingering economic distress.

The maximum marginal U.S. income tax rate of 35 percent applies — in theory more than practice — to households with taxable income of over about $388,500.

Like many wealthy people, the Romneys have been helped by changes in federal tax policy that have placed much lower tax rates on investment income — from dividends, interest and capital gains from the sale of stocks and other assets — than on wages and salaries, the source of income for most Americans.

Under the Bush-era tax cuts strongly supported by most Republicans, such investment income, including gains on securities held for a year or longer, is subject to a tax rate of 15 percent.

According to the congressional Joint Committee on Taxation, an average federal tax rate of 15 percent — including both income and payroll taxes — would apply to households with taxable incomes of from $75,000 to $100,000.

Obama and his wife paid federal taxes of just over 25 percent of their 2010 income of $1.7 million, mostly from the books he's written.

Texas Gov. Rick Perry and his wife paid roughly 24 percent of their 2010 income of $217,447.